Phone receives updates, with the same frequency and method of delivery as would be the case on global OnePlus running oxygen os. There's also the option to use 3rd party app such as oxygen updater to download full or incremental updates when they become available.
What is your area of origin? Accessories are almost none existent locally, locally being South Africa?
Having said that I have never had an issue importing covers, charging bricks, cables and replacement batteries for an aging OnePlus 7T pro. A couple of years ago I bought 3 batteries off of AliExpress, original OP batteries. R1.2k all inclusive.
A warranty claim would be potentially a far more complex process. The phone would need to be shipped back to OnePlus China I would assume. Time delays, risk and cost all incurred by the owner.
I've been a OnePlus user since the 3T back in 2016. Have owned 6 or 7 OnePlus phones since and not 1 days issue with any of them. Not one!!
Having said that I have always been worried about an OBF. Such an event would be unfortunate and rather inconvenient.